1. Regarding THP
Tomlinson Harashima Precoding (THP) is a technique in which under a situation where interference exists, a transmitter previously knows the interference, previously cancels the interference from a transmission signal and transmits the signal to a receiver. In this process, this method causes both the transmitter and the receiver to carry out a Modulo (remainder) operation to transmit and receive the signal in which an increase in transmission power due to the cancellation of the interference is suppressed (refer to later mentioned non-patent document 1).
A description will be given of the Modulo operation carried out by both the transmitter and the receiver in the communication using THP. The Modulo operation is a process of reducing transmission power by keeping the amplitude of a transmission signal within a certain range or less. To be specific, the Modulo operation is an operation in which both the transmission and reception sides add a signal being an integer multiple of a known value τ to an I-ch (In-phase channel) and a Q-ch (Qadrature channel) of the transmission signal by both the transmission and reception sides, and thereby convert the transmission signal into a signal within a range of [−τ/2, τ/2]. An example of the Modulo operation is shown in FIG. 24. In FIG. 24, the Modulo operation is shown as a process of shifting a signal represented as ● to a position of ◯, and of shifting ● to ∘ by adding a perturbation vector d(=(−2)τ+j*(−1)τ) to ●. Here, j represents an imaginary unit. Both the I-ch and the Q-ch of ◯ are within the range between the origin and [−τ/2, τ/2]. Thus, the Modulo operation is effective in keeping the amplitude of a signal within a certain range. Generally, if mean power of a modulation symbol is normalized to 1, a Modulo width τ is a certain value previously known by the transmission and reception sides, according to the modulation scheme. For example, τ=2√2 in QPSK, τ=8/√10 in 16QAM, and τ=16/√42 in 64QAM.
This Modulo operation enables transmission of a signal in which the increase in transmission power due to removal of interference is suppressed, even in an environment where the reception side undergoes a large interference. The Modulo operation is expressed as:
                    [                  Formula          ⁢                                          ⁢          1                ]                                                                                  Mod            τ                    ⁡                      (            x            )                          =                  x          -                                    floor              (                                                                    Re                    ⁡                                          (                      x                      )                                                        +                                      τ                    2                                                  τ                            )                        ⁢            τ                    -                                    j              ·                              floor                (                                                                            Im                      ⁡                                              (                        x                        )                                                              +                                          τ                      2                                                        τ                                )                                      ⁢                          τ              .                                                          (        1        )            
Here, j represents an imaginary unit, Re(x) represents a real part of x, and Im(x) represents an imaginary part of x. In addition, floor(x) represents the largest integer below x.
Next, a principle of THP will be described. Assume that s is a desired signal and f is interference. The transmitter firstly subtracts interference f from desired signal s because THP is based on the assumption that interference f is previously known. However, since the signal s−f resulting from the subtraction normally has a large amplitude, transmission power will be increased if the signal is transmitted as it is. For this reason, the transmitter carries out the Modulo operation on the signal s−f and transmits the resultant signal expressed as Modt(s−f).
With this operation, the transmitter can keep the I-ch and the Q-ch of the transmission signal within the range between the origin and [−τ/2, τ/2], and thereby transmit a signal with less power than the case of transmitting the signal s−f. Here, assuming that a characteristic of a channel is 1 and ignoring the influence of noise, a reception signal is expressed as Modt(s−f)+f because the receiver undergoes interference f. By carrying out the Modulo operation on this reception signal, the receiver can detect the desired signal as in the following formula:
[Formula 2]Modτ(Modτ(s−f)+f)=Modτ(s−f+f)=Modτ(s)=s   (2)
By thus carrying out the Modulo operation on the reception side as well, the desired signal s can be reconstructed on the reception side. The above is the mechanism of THP.
2. MU-MIMO THP
(Overall System Configuration)
As shown in FIG. 25, when a base station (BS) transmits signals to multiple mobile terminals (MTs) at the same time point at the same frequency, a multi-user interference (MUI) occurs usually. Downlink (DL) MU-MIMO (Multi-User Multi Input Multi Output) is a method of using THP to cancel the MUI with high power efficiency, and multiplexing multiple MTs.
DL MU-MIMO THP is a technique based on the premise that the BS knows all channel state information (CSI) of the MTs. This is because THP requires, as described above, that the BS being the transmitter know the interference that the MT being the receiver undergoes, and DL MU-MIMO THP requires that the CSI be used to calculate the interference.
Hereinbelow, a description will be given of configurations of a BS and MT in the DL MU-MIMO THP with reference to the drawings. Although a case of two MTs is used herein to simplify the description, a case of multiplexing any number of MTs by MU-MIMO THP can be implemented likewise (refer to later mentioned non-patent document 2).
(BS Configuration (2MTs))
The BS knows CSI for each of the MTs, and simultaneously transmits signals to two MTs at the same time point at the same frequency. At this time, as shown in FIG. 25, in order to prevent the two MTs (MT1, MT2) from interfering with each other, two types of interference affecting each other including interference by signals for MT1 interfering with MT2, and interference by signals for MT2 interfering with MT1 need to be cancelled. The BS cancels one of the two types of interference by THP, and cancels the other by multiplying the interference by a linear filter. FIG. 26 shows a concrete configuration example of a BS for achieving communication with the two MTs. Hereinafter, a description will be given of a configuration of the BS according to the exemplar configuration shown in FIG. 26.
(Configuration of BS)
First of all, encoders 101-1, 2 convert information bits for the respective MTs into error correcting codes, and input the coded bits for the respective MTs to modulators 103-1, 2. The modulators 103-1, 2 modulate the coded bits for the respective MTs inputted thereto, and generate modulated signals for the respective MTs. After generating the modulated signal for MT1, the modulator 103-1 inputs a modulation symbol for MT1 to an interference calculator 113 and to a linear filter multiplier 115. After generating the modulated signal for MT2, the modulator 103-2 inputs a modulation symbol for MT2 to an interference subtractor 107.
A linear filter calculator 117 calculates a linear filter and interference coefficient information by use of CSI known to the BS, and inputs them to the linear filter multiplier 115 and the interference calculator 113, respectively. Then, the interference calculator 113 calculates the interference that MT2 undergoes, by use of the interference coefficient information as well as the modulated signal inputted from the modulator 103-1 having generated the modulated signal for MT1, and inputs the interference to the interference subtractor 107. The interference subtractor 107 subtracts the interference that MT2 undergoes from the modulated signal for MT2, and then inputs the signal after the subtraction to a Modulo operation part 111. The Modulo operation part 111 carries out the Modulo operation shown in formula (1) on the signal after the subtraction, and inputs the signal after the Modulo operation to the linear filter multiplier 115. In FIG. 26, the interference subtractor 107, the interference calculator 113 and the Modulo operation part 111 surrounded with a broken line are referred to as a nonlinear spatial multiplexer 105.
The linear filter multiplier 115 multiplies each of the inputted signal for MT1 and signal for MT2 by a linear filter. With this operation, a Null of the signal for MT2 is directed toward MT1, so that MT1 can be free from the interference of the signal for MT2.
Thereafter, the linear filter multiplier 115 inputs the signals after the linear filter multiplication to transmitters 121-1, 2. The transmitters 121-1, 2 perform digital-to-analog conversion on the signals after the linear filter multiplication, upconvert the signals to a carrier frequency and transmit the resultant signals to MT1 and MT2, respectively.
(MT Configuration (2MTs))
MT1 and MT2 receive signals transmitted from the BS. Each MT performs reception processing on the reception signal by carrying out the same Modulo operation as the BS. The MT will be described in detail with reference to FIG. 27.
(Configuration of MT)
A receiver 131 downconverts a signal received by an antenna AT from the carrier frequency to baseband and performs analog-to-digital conversion on the signal to generate a baseband digital signal. Then, the receiver 131 inputs the baseband digital signal to a channel compensator 133. The channel compensator 133 performs channel compensation on the baseband digital signal, and inputs the signal after the channel compensation to a Modulo operation part 135. The Modulo operation part 135 carries out the Modulo operation shown in formula (1) on the signal after the channel compensation, and inputs the signal after the Modulo operation to the demodulator. The demodulator 137 demodulates the signal after the Modulo operation and inputs the demodulation result to a decoder 141.
(Explanation of Theory)
As has been described with reference to FIG. 26 and FIG. 27, the mechanism of DL MU-MIMO THP includes: using THP to remove one of two types of interference caused by signals for MT1 and MT2 interfering with each other, and removing the other by multiplying the interference by a linear filter. Hereinafter, this mechanism of DL MU-MIMO THP will be described in detail from a theoretical aspect.
(Definition of Variable)
Assume that h11, h12 represent complex gain of channels from two antennas of the BS to MT1. Similarly assume that h21, h22 represent complex gain of channels to MT2. Using these values, a channel matrix H is expressed as:
                    [                  Formula          ⁢                                          ⁢          3                ]                                                            H        =                              (                                                                                h                    11                                                                                        h                    12                                                                                                                    h                    21                                                                                        h                    22                                                                        )                    .                                    (        3        )            
In addition, assume that s1 and s2 are modulated signals for MT1 and MT2, respectively.
(Linear Filter Calculation)
The linear filter calculator 115 uses a linear filter to direct a null of a signal for MT2 toward MT1, thereby to cancel interference by signals for MT2 interfering with MT1. The linear filter calculator 115 obtains this linear filter by performing QR decomposition on the Hermitian conjugate HH of the channel matrix H. QR decomposition is a method of decomposing a given matrix into a product of a unitary matrix Q and an upper triangular matrix R, and HH after the QR decomposition is expressed as:
[Formula 4]HH=QR  (4).
Here, each of Q and R is a matrix including two rows and two columns, and R is an upper triangular matrix in which a component of second row first column is 0. The linear filter to be used in the multiplication by the linear filter calculator 115 is the unitary matrix Q of formula (4). When HQ of a combination of the linear filter Q and the actual channel matrix H is considered as an equivalent channel, HQ is expressed as:
[Formula 5]HQ=(QR)HQ=(RHQH)Q=RH  (5).
As R is an upper triangular matrix, RH is a lower triangular matrix. Specifically, a component of first row second column of the equivalent channel RH is 0. Assuming that noise is 0, respective reception signals y1, y2 of MT1 and MT2 can be calculated as:
                    [                  Formula          ⁢                                          ⁢          6                ]                                                                      (                                                                      y                  1                                                                                                      y                  2                                                              )                =                                            R              H                        ⁡                          (                                                                                          s                      1                                                                                                                                  s                      2                                                                                  )                                =                                                    (                                                                                                    r                        11                        *                                                                                    0                                                                                                                          r                        12                        *                                                                                                            r                        22                        *                                                                                            )                            ⁢                              (                                                                                                    s                        1                                                                                                                                                s                        2                                                                                            )                                      =                                          (                                                                                                                              r                          11                          *                                                ⁢                                                  s                          1                                                                                                                                                                                                                              r                            12                            *                                                    ⁢                                                      s                            1                                                                          +                                                                              r                            22                            *                                                    ⁢                                                      s                            2                                                                                                                                              )                            .                                                          (        6        )            
Here, a component of row k column I of R is expressed as rk1. In addition, * indicates a complex conjugate. It can be seen from formula (6) that y1 does not include an s2 component. To be specific, the BS multiplies signals for MTs by the linear filter Q to prevent signals for MT2 from reaching MT1. In other words, The BS directs the null of the signal for MT2 toward MT1.
(Interference Calculation)
Having cancelled the interference by signals for MT2 interfering with MT1 by use of the linear filter, interference by signals for MT1 interfering with MT2 will next be cancelled by use of afore-mentioned THP.
As shown in formula (2), firstly, interference f needs to be calculated in order for the BS to perform THP. In this case, the value is found by assuming that f is interference after MT2 has performed channel compensation. According to formula (6), a reception signal z2 having undergone the channel compensation by MT2 is expressed as:
[Formula 7]z2=1/r*22·y2=1/r*22·(r*12s1+r*22s2)=s2+r*12/r*22·s1  (7)
Since the interference component f indicates terms other than a signal s2 for MT2, the interference is expressed as:
[Formula 8]f=r*12/r*22·s1  (8)
Here, f is obtained by multiplying a signal s1 for MT1 by coefficient r*12/r*22. This coefficient r*12/r*22 represents the interference coefficient information. The linear filter multiplier calculates the interference coefficient information and inputs the result to the interference calculator, and then the interference calculator calculates the interference f shown in formula (8) with the coefficient r*12/r*22 and the modulated signal s1 for MT1.
(THP)
The BS calculates a signal Modt(s2−f) by use of the interference f and the modulated signal s2 for MT2 in the interference subtractor 107 and the Modulo operation part 111, and inputs the signal to the linear filter multiplier 115. The linear filter multiplier 115 multiplies each of the signal Modt(s2−f) and the signal s1 for MT1 by the linear filter Q and calculates a transmission signal. In this configuration, the interference subtractor 107, the interference calculator 113 and the Modulo operation part 111 shown in FIG. 26 are referred to as the nonlinear spatial multiplexer 105. In addition, the process of calculating the signal to be inputted to the linear filter multiplier 115 by carrying out the interference cancellation and the Modulo operation on the modulated signal is referred to as nonlinear-spatial multiplexing.
(Reception Signal)
Replacing s2 with Modt(s2−f) in formula (6), the reception signal is found as:
                    [                  Formula          ⁢                                          ⁢          9                ]                                                                                                                (                                                                                                    y                        1                                                                                                                                                y                        2                                                                                            )                            =                            ⁢                                                HQ                  (                                                                                                              s                          1                                                                                                                                                              Mod                          ⁡                                                      (                                                                                          s                                2                                                            -                              f                                                        )                                                                                                                                )                                =                                                      R                    H                                    ⁡                                      (                                                                                                                        s                            1                                                                                                                                                                            Mod                            ⁡                                                          (                                                                                                s                                  2                                                                -                                f                                                            )                                                                                                                                            )                                                                                                                          =                            ⁢                                                (                                                                                                                                          r                            11                            *                                                    ⁢                                                      s                            1                                                                                                                                                                                                                                                  r                              12                              *                                                        ⁢                                                          s                              1                                                                                +                                                                                    r                              22                              *                                                        ·                                                          Mod                              ⁡                                                              (                                                                                                      s                                    2                                                                    -                                  f                                                                )                                                                                                                                                                                          )                                .                                                                        (        9        )            
Here, MT2 carries out the Modulo operation after the channel compensation of the reception signal y2. Then, as shown in
                    [                  Formula          ⁢                                          ⁢          10                ]                                                                                                                Mod                ⁡                                  (                                                            1                      /                                              r                        22                        *                                                              ·                                          y                      2                                                        )                                            =                            ⁢                              Mod                ⁡                                  (                                                                                                              r                          12                          *                                                /                                                  r                          22                          *                                                                    ⁢                                              s                        1                                                              +                                          Mod                      ⁡                                              (                                                                              s                            2                                                    -                                                                                                                    r                                12                                *                                                            /                                                              r                                22                                *                                                                                      ⁢                                                          s                              1                                                                                                      )                                                                              )                                                                                                        =                            ⁢                              Mod                ⁡                                  (                                                                                                              r                          12                          *                                                /                                                  r                          22                          *                                                                    ⁢                                              s                        1                                                              +                                          (                                                                        s                          2                                                -                                                                                                            r                              12                              *                                                        /                                                          r                              22                              *                                                                                ⁢                                                      s                            1                                                                                              )                                                        )                                                                                                        =                            ⁢                              s                                  2                  ,                                                                                        (        10        )            MT2 is able to detect the modulated signal s2 directed thereto. By similarly carrying out the Modulo operation after channel compensation, the modulated signal for MT1 can also be found.
                    [                  Formula          ⁢                                          ⁢          11                ]                                                                      Mod          ⁡                      (                                          1                /                                  r                  11                  *                                            ·                              y                1                                      )                          =                              Mod            ⁡                          (                                                                    r                    11                    *                                    /                                      r                    11                    *                                                  ⁢                                  s                  1                                            )                                =                                    Mod              ⁡                              (                                  s                  1                                )                                      =                                          s                1                            .                                                          (        11        )            
As described above, both MT1 and MT2 are able to detect the signals directed to themselves.
3. Multistream Communication with MTs Having Multiple Reception Antennas
The above description has been given of a method of transmitting a single datastream at a time to different MTs. Meanwhile, as shown in FIG. 28, there is also a technique of using MU-MIMO THP to spatially multiplex MTs at the same time point at the same frequency, the MTs each having multiple reception antennas and performing multi-datastream communication by SU-MIMO (refer to non-patent document 3). With this technique, spatial resource can be used efficiently for MTs having multiple reception antennas as well. Hence, even when multi-stream communication is performed, the BS transmits a datastream for each MT after carrying out a Modulo operation thereon.
4. Regarding DRS
In DL MU-MIMO THP, dedicated reference signals (DRSs) for respective MTs used for demodulation are transmitted not by spatial multiplexing but by a multiplexing method in which the DRSs for MTs are divided and allocated to orthogonal radio resources (such as time-division multiplexing and frequency-division multiplexing) (refer to patent document 1). Hereinafter, each of the DRSs for MTs divided and allocated to orthogonal radio resources is referred to as an “orthogonal DRS.” The orthogonal DRS is used because: if the BS transmits the DRS by spatial multiplexing using MU-MIMO THP like in transmitting a data signal configured of a signal of modified information bits and the like, the MT cannot carry out a Modulo operation on the DRS since the amplitude of the signal is unknown, and cannot perform channel estimation based on the transmitted DRS after the Modulo operation.
In the case of transmitting the DRS to each MT with an orthogonal radio resource, the BS transmits the DRS previously known to both the BS and the MT (represented as a complex number q on a signal plane). At this time, the BS transmits the DRS to only one MT with a single orthogonal radio resource, and does not transmit signals to other MTs. Accordingly, the MT can receive the DRS transmitted from the BS without being affected by interference, and can divide a reception signal point y of the DRS by q to obtain a channel h=y/q.
On the other hand, if the BS simultaneously transmits the DRSs to multiple MTs by spatial multiplexing using THP as in the case of transmitting a data signal, the MT cannot perform channel estimation. This is because the BS transmits the DRS after carrying out a Modulo operation thereon, and thus the signal to which the MT should refer is not q but the signal q with a perturbation vector d added thereto by the Modulo operation. Specifically, the reference signal is a point represented by q+d, and although the MT should divide the reception signal point y by q+d, the MT is unable to know the value of d in advance. Hence, the MT cannot estimate the value of channel h.
For this reason, DL MU-MIMO THP uses the orthogonal DRS instead of the method of spatially multiplexing the DRSs for the MTs (refer to later mentioned patent document 1).